The number of resistance genes available to date for cloning in Streptomycetes is relatively small, and some of them are additionally subject to special restrictions. Thus, for example, hygromycin is highly toxic, viomycin is no longer commercially available, and chloramphenicol is suitable only in special cases because of the known instability of the resistance gene in Streptomycetes. Thus there is a need for further resistance markers.
Many Streptomycetes are extremely sensitive to the antibiotic gentamicin. Thus, as a rule, growth on the plate is inhibited at a gentamicin concentration of only 0.5 .mu.g/ml.